Index-guided semiconductor lasers have proven to be useful devices. Index guiding refers to the use of variations in the optical index of refraction between material layers to provide for optical confinement. To fabricate an index-guided semiconductor laser one must use a suitable material system, create an active region, and provide for carrier confinement, radiation confinement, and optical feedback. Modern semiconductor lasers may fulfill these requirements using a heterostructure formed in a suitable material system (such as AlGaInP, AlGaAs, or InGaAsP), a lateral waveguide which provides for optical confinement, and an optical Fabry-Perot cavity formed from cleaved facets.
The following uses the subscripted notation, A.sub.M L.sub.N, where the nonsubscript terms (A and L) designate a material (be it an element or a compound) and the subscripted terms (M and N) designate the atomic concentration of their associated material. Additionally, parenthesis are used to identify compounds which act together. For example, the notation (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P implies that half of the material is phosphorous, one quarter is comprised of indium, and the remaining quarter is a compound of aluminum and gallium in which the composition takes a range (0.ltoreq.X.ltoreq.1) from all aluminum (the subscripted X is equal to 1) to all gallium (the subscripted X is equal to 0). It is to be understood that the subscripted terms are, in practical materials, only approximations. For example, in a material designated as (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P, the subscripted percentages X and 0.5 may vary slightly (such as by .+-.0.05). The general alloy (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P) will be denoted AlGaInP for convenience.
An important semiconductor laser material system, and one which is particularly useful for fabricating visible light emitting semiconductor lasers, is (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P. Importantly, (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P both lattice matches with GaAs substrates and can be grown using organometallic vapor phase epitaxy (OMVPE). Interestingly, OMVPE-grown AlGaInP exhibits two distinct phases: an ordered phase, (consisting of a monolayer InP/Al.sub.x Ga.sub.1-x P superlattice formed by spontaneous atomic organization during growth) characterized by a lower bandgap; and a random-alloy phase, one with no ordering on the group III sublattice, having a higher bandgap energy. Importantly, the lower bandgap phase occurs when AlGaInP is OMVPE grown on (001) orientated substrates, while the higher bandgap phase occurs when AlGaInP is OMVPE grown on substrates whose surfaces are misorientated from (100) toward (111)A, such as at angles of 5 to 55 degrees (the 111 plane).
One possible way of implementing AlGaInP index-guided semiconductor lasers is to use ridged substrates. In such implementations, lateral optical and electrical confinement results from the refractive index difference and the bandgap energy difference, respectively, between the ordered and random phases of AlGaInP grown on a ridge. However, ridged semiconductor lasers typically have a relatively large planar surface from which relatively small mesas protrude. This is disadvantageous when attempting to heatsink ridged semiconductor lasers since the mesas interfere with the direct mounting of the device on a planar heatsink.
It would be useful to have index-guided semiconductor lasers which achieve index-guiding using phase differences which result from the growth of material on different crystalline planes. Beneficially, such lasers would be directly mountable on a planar heatsink.
A major problem when attempting to grow lattice-matched materials on different crystal planes are composition differences, which introduce strain. Since different AlGaInP compositions generally result from OMVPE growth on different crystal planes, it is difficult to prepare dislocation-free (lattice-matched) materials on structured substrates. Strain limits the usable thickness of a lattice-mismatched epitaxial layer. For example, when growing AlGaInP on GaAs to achieve a desired alloy composition it is critically important to ensure lattice-matched layers (unstrained, and free of dislocations). For a mismatched layer (whose composition does not match the lattice parameter of GaAs), if the critical thickness is exceeded, misfit dislocations appear which significantly degrade the optical and electrical properties of the resulting semiconductor layers.